In telephony, the term in-band signaling generally means the inclusion of control signals in the same communication medium with traffic signals. Most telephone signaling is still done out of band, for example rotary dialing, which requires a separate E lead and M lead in addition to the voice traffic line. An example of in-band signaling in telephony is multi-frequency dialing wherein tone frequencies characteristic of dialing digits are interposed on the same communication line with voice signals.
The conventional means for establishing a switched data call is the use of an auto-call unit in combination with a modem, both of which are connected in parallel between a data terminal equipment and the communication channel. During the call connection signaling portion of the session, the data terminal equipment sends digits to the auto-call unit which converts them to dialing pulses which are transmitted over the communication channel, thereby establishing the call connection. After the call connection has been established, control is released to the modem, which converts the signals from the data terminal equipment to suitable signals for transmission over the communication link, which starts the traffic portion of the session. During the call connection period, the data terminal equipment (DTE) sends multiple-bit digits to the auto-call unit which in turn sends corresponding dial-pulse sequences on the telephone line. After selection and ringing, the auto answer unit seizes the line and connects the modem to the line. At that time, the modem sends an answer tone which constitutes the beginning of a handshaking protocol. After a time-out or after the detection of the tone, the calling auto-call unit switches the line to the calling modem and a handshaking protocol takes place between the terminals. Conventional auto-call units employ the EIA standard RS-366 which provides for an interface between a data terminal equipment and auto-calling equipment for data communication.
Still another type of control signaling connection for a data terminal equipment to a communications controller is the use of an associated voice port along with the digital data port, to establish the call connection. To each switched digital data port there is associated a voice port and a line. The call connection dialing is carried out on the voice port line as is done for a normal voice call. A comparative table in the communications controller associates the voice port line with the digital data port. Upon completion of the selection process through the associated voice port, the digital data port is connected to the remote digital data port at the other end of the communication link for the traffic portion of the session. There is no voice traffic on the associated voice port line and no traffic-handling resources are allocated to the voice port for the call. The problem with this approach is that it requires the dedication of one voice port for each digital data port, which is a costly waste of the voice port resources.